In total knee joint replacement surgery or arthroplasty, the knee joint is replaced with an artificial implant. The implant typically includes a tibial component, a femoral component, and a mobile bearing insert (a patella component).
Most femoral components have an inner surface with intersecting flat surfaces that interface with a surgically prepared distal femoral surface. The outer surface typically has a curvature that faces a corresponding tibial component attached to the patient's tibia. Two curved condylar regions on the femoral component replicate the natural femoral condyles.
Determining the proper anterior/posterior (A/P) size of the distal femur is a difficult procedure. The surgeon must measure the correct A/P size of the distal femur, while setting rotational alignment to anatomic landmarks (e.g., epicondylar axis and the A/P axis). The A/P femoral size is usually derived from the distance between the posterior condyles and the anterior cortex of the distal femur. The rotational alignment (rotation on a transverse plane at the distal face of the femur) is usually derived from anatomic landmarks. This rotational alignment, however, is typically not in perpendicular/parallel relation or at a set angle to the A/P measurement plane needed to properly size the distal femur. Accordingly, it is not as easy as measuring from the back (posterior) to the front (anterior) of the bone and choosing an implant.
Instruments used for this sizing in the past have included two stationary paddles that are placed vertically against the posterior surface of a resected distal femur. These instruments do not account for or measure rotational alignment between two boundaries (for alignment with anatomic landmarks), while still referencing both posterior condyles to determine the proper A/P size of the distal femur. Stationary paddles also do not account for condyles of different sizes and shapes (e.g., one condyle may extend further than another due to patient irregularity or diseased tissue).
However, instruments that address each variable (rotation and size) independently increase procedural time and, more importantly, can result in less than optimal A/P sizing. For example, the use of two different instruments prevents the sizing from being measured in relation to the rotational location being used. Specifically, the sizing is not measured perpendicular to the rotational location being used.
Some instruments do measure both rotational alignment and A/P sizing simultaneously. However, these instruments lack adjustment, so that when they are rotated with respect to the anatomic landmarks, they only reference one posterior condyle. (Again, a precise measurement is not obtainable because one condyle may extend further than another due to patient irregularity or diseased tissue or any other reason.) In other cases, the instruments have additional components that can be added to the instrument to substitute for condylar contact, but these additional components are in increments that limit adjustability and can add to surgical procedure time because they need to be interchanged to determine “best fit” scenarios.
Accordingly, there is a need to provide a femoral sizing guide that measures and correlates both (1) rotational alignment (the rotation at the distal face of the femur) and A/P sizing.